System VERY slow - Processor usage constant 100%

Everything is running very slow, mouse cursor is slow to respond and a delay
in entering text from the keyboard, programs slow to open. Checked with
system monitor, it showed kernel processor usage at 100% - constantly.

I have rebooted and closed unnecessary programs, and monitor will show 5%,
11%, even 20% at max. But after a few minutes or after opening a program
(like Word, Access, MSN or IE) it goes right back to a VERY slow response. I
checked System Information: running tasks appeared normal, everything else
appeared normal; however, problem devices showed error code 22 for the device
'ACPI IRQ Holder for PCI IRQ Steering'. I've got no idea what that is.

PLEASE HELP!!!